Corney, D, Haynes, J, Rees, G, & Lotto, RB : The Brightness of Colour. PLoS ONE
(2009)

Background: The perception of brightness depends on spatial context: the same stimulus can appear light or dark depending on what surrounds it. A less well-known but equally important contextual phenomenon is that the colour of a stimulus can also alter its brightness. Specifically, stimuli that are more saturated (i.e. purer in colour) appear brighter than stimuli that are less saturated at the same luminance. Similarly, stimuli that are red or blue appear brighter than equiluminant yellow and green stimuli. This non-linear relationship between stimulus intensity and brightness, called the Helmholtz-Kohlrausch (HK) effect, was first described in the nineteenth century but has never been explained. Here, we take advantage of the relative simplicity of this ‘illusion' to explain it and contextual effects more generally, by using a simple Bayesian ideal observer model of the human visual ecology. We also use fMRI brain scans to identify the neural correlates of brightness without changing the spatial context of the stimulus, which has complicated the interpretation of related fMRI studies.

Results: Rather than modelling human vision directly, we use a Bayesian ideal observer to model human visual ecology. We show that the HK effect is a result of encoding the non-linear statistical relationship between retinal images and natural scenes that would have been experienced by the human visual system in the past. We further show that the complexity of this relationship is due to the response functions of the cone photoreceptors, which themselves are thought to represent an efficient solution to encoding the statistics of images. Finally, we show that the locus of the response to the relationship between images and scenes lies in the primary visual cortex (V1), if not earlier in the visual system, since the brightness of colours (as opposed to their luminance) accords with activity in V1 as measured with fMRI.

Conclusions: The data suggest that perceptions of brightness represent a robust visual response to the likely sources of stimuli, as determined, in this instance, by the known statistical relationship between scenes and their retinal responses. While the responses of the early visual system (receptors in this case) may represent specifically the statistics of images, post receptor responses are more likely represent the statistical relationship between images and scenes. A corollary of this suggestion is that the visual cortex is adapted to relate the retinal image to behaviour given the statistics of its past interactions with the sources of retinal images: the visual cortex is adapted to the signals it receives from the eyes, and not directly to the world beyond.

Download

Clarke, R. and Lotto, R.B.: Visual processing of the bee innately encodes higher-order image statistics when the information is consistent with natural ecology. Vision Research.
(2009)

Determining the statistical relationships of images that facilitate robust visual behaviour is nontrivial. Here we ask if some spatial relationships are more easily learned by the visual brain than others. Visually naïve bumblebees were trained to recognise coloured artificial flowers in scenes of equal spatial complexity but differing patterns of stimulus intensity. When flowers of similar intensity were grouped into extended regions across the array (coincident with natural patterns of light), the accuracy of the bees' foraging behaviour was dependent on spatial context, even though this information was redundant to the task. When the same intensity information was organised into a pattern that was less consistent with natural patterns of illumination but of equal order, their behaviour was independent of spatial context and they required double the training time to solve the same conditional task. These observations suggest the brain is biased to more efficiently encode/learn ecologically ‘meaningful' image correlations.

Download

Alvarez-Buylla, E.R., Benitez, M., Chaos, A., Cortes-Poza, Y., Espinosa-Soto, C. Lotto, R.B., Malkin, D., Escalera-Santos, G.J. and Padilla-Longoria, P.: Floral morphogenesis: Stochastic exploration of a gene network's epigenetic landscape. Public Library of Science 3:e3626.
(2008)

Fundamental to understanding the evolution and function of biological systems is to comprehend how such systems develop. Most consider the process of development to be a wholly hierarchical genetically programmed process. Without dismissing the importance of genetic regulation, here we consider the hypothesis that development, at least of some systems, can be understood as an epigenetic landscape (first proposed by Waddington, 1957), where the contours reflect the attractor states of the system's regulatory network(s). We specifically study as a stochastic system the network that underlies cell fate determination for floral organ primordia in Arabidopsis thaliana, which is a key system in plant biology research. Arabidopsis, like most angiosperms, follows a stereotypical sequence in the spatial and temporal differentiation of its floral primordia cells: first to sepals, then to petals, then to stamens and ultimately to carpels. Also well-known are fifteen of the genes that are involved in a complex regulatory network underlying this developmental process. Here we propose a method to reduce the dimensionality of genetic networks using both discrete and continuous stochastic models of cell populations, which attain each of the four primordial states (sepals, petals, stamens and carpels) with the same temporal sequence observed in Arabidopsis flower development. Thus our results suggest that such temporal sequence is, at least in part, a robust emergent consequence of the interactions in the gene network. Therefore, our results could also explain the evolutionary conservation of angiosperm flower patterning despite the contrasting conditions in which plants of diverse lineages develop. More generally, the computational and conceptual framework presented here provides the basis for linking genes, morphogenetic patterns and the evolution of any dynamical system or network. 

Bergstrom and Lotto: Harnessing the Enactive Knowledge of Musicians to Allow the Real-Time Performance of Correlated Music and Computer Graphics - Leonardo Journal (in press)
(2008)

Artists and scientists have a perpetual interest in the relationship between music and art. As technology has progressed, so too have the tools that allow the practical exploration of this relationship. Today, artists in many disparate fields occupy themselves with producing animated visual art that is correlated with music (called ‘visual music'). Despite this interest and advancing technology, there still is no tool that will allow one to perform visual music in real-time with a significant level of control. Here we propose a system that would enable a group or individual to perform live ‘visual music' using the musical instrument(s) itself as the primary source of control information for the graphics.

Le Martelot, E., Bentley, P. and Lotto, R.B.: Eating Data is Good for Your Immune System: An Artificial Metabolism for Data Clustering using Systemic Computation. Refereed Proceedings: Proceedings of International Conference on Artificial Immune Systems (ICARIS 2008), Phuket, Thailand.
(2008)

Previous work suggests that innate immunity and representations of tissue can be useful when combined with artificial immune systems. Here we provide a new implementation of tissue for AIS using systemic computation, a new model of computation and corresponding computer architecture based on a systemics world-view and supplemented by the incorporation of natural characteristics. We show using systemic computation how to create an artificial organism, a program with metabolism that eats data, expels waste, clusters cells based on the nature of its food and emits danger signals suitable for an artificial immune system. The implementation is tested by application to a standard machine learning set and shows excellent abilities to recognise anomalies in its diet.

Download

Le Martelot, E., Bentley, P. and Lotto, R.B.: Crash-Proof Systemic Computing: A Demonstration of Native Fault-Tolerance and Self-Maintenance. Refereed Proceedings: Proceedings of Advances in Computer Science and Technologies (ACST 2008), Langkawi, Malaysia.
(2008)

Reliability in computer or engineering systems is undoubtedly a key requirement in the development process. Safety within critical control systems, and reliable data transfers, require tolerance to unexpected and unwanted phenomena. In biology, new cells can replace damaged cells, DNA is able to repair and replicate with error control. These processes are essential to maintain the overall organism. Biology has often been a successful inspiration in computation (artificial neural networks, genetic algorithms, ant colony optimisation, etc) although conventional computation differs widely from natural computation. In this respect, we have introduced systemic computation (SC), a model of interacting systems with natural characteristics and suggested a new computer architecture. Following this work, we then introduced a systemic computer as a virtual machine running on conventional computers. In this paper we show, using a genetic algorithm implementation running on this platform, how crash-proof programs following the SC paradigm have native fault-tolerance and easily integrated self-maintenance. 

Download

Bergstrom and Lotto: Making the Performance of Real-Time Computer Graphics Accessible to Non-programmers. Referee Proceedings - (re)Actor.
(2008)

Here we describe a programme that will enable the performance of advanced real-time computer graphics by non-programmers through flexible modules that are easily exchanged and managed. 

Download

Lotto, R.B., Bex, P. and Ip, B.: Visual attention: Biasing perception towards the statistics of the attended stimulus feature. (Submitted)
(2008)

Some images are perceptually bi-stable because the image's global – or average – description is ambiguous: when considered as a whole such images have multiple, equally likely ‘interpretations' between which visual consciousness seems to fluctuate. While these fluctuations are thought to be random, here we consider whether in some instances they represent coherent shifts in attention. To test this, subjects viewed an ‘X' moving behind an aperture. When attention (and eye-movements) were unconstrained, subjects reported seemingly random fluctuations in their conscious awareness between an ‘X' moving back-and-forth across the aperture (pattern motion), and two bars moving in opposition along the aperture's vertical axis (component motion). The timing and duration of their fluctuations were unpredictable within and between subjects, thus demonstrating the inherent bi-stability of the global stimulus. Subjects were then asked to attend to local features of the X. When attending its centre – where the local motion energy is vertical, they reported only pattern motion. Conversely, when attending the end points of the ‘X' – where the local motion energy is horizontal, they perceived only component motion. In this instance, then, visual awareness of a globally ambiguous image is not random, but perfectly predicted by the statistics of the attended element. What we see, then, is determined by the current topology of a high-dimensional feature landscape in the visual cortex, which itself represents, not only evolutionary and developmental history, but also one's immediate attentional state. 

Wicklein, M. and Lotto, R.B.: Bees innately encode context. Philosophical Transactions of the Royal Academy. (under review).
(2008)

Previous research has shown that bees generate colour constancy by learning the spectral relationships in scenes that were useful in the past. Here we consider whether bees encode contextual relationships even in invariant environments where relational information is redundant. Under these constrained conditions, bees still used relational information to recognise the rewarding stimulus. However, the same bees also remembered the rewarding stimulus in absolute terms, independent of context. We therefore conclude that bumblebees can use and ignore seeming mutually exclusive visual information simultaneously to generate behaviour, thus maximising robustness (in line with their phylogenetic experience) and computational efficiency with respect to the statistics of their extant visual environment. 

Lotto, R.B.: Statistical analysis of visual perception, in The New Encyclopedia of Neuroscience, edited by Larry Squire et al. In press.
(2008)

While it is possible to describe a code in great detail, deciphering it requires understanding the nature of the information encoded. Similarly, explaining how we see what we do requires not only quantitative descriptions of the visual brain's functional architecture, but also a clear understanding of what is represented in that architecture. Theoretical and computational neuroscience attempts to explain what that information might be. Here I review the rationale and evidence for the hypothesis that the brain encodes the statistics of natural images and the behavioural significance of natural images in past visual experience. 

Lotto, R.B.: Using illusions to teach children about the science and art of seeing and conceiving in Creative Encounters. Wellcome Books. In press
(2008)

An illusion is the phenomenon of perceiving something different from what is physically there. This chapter will describe why illusions may be a useful tool in the classroom for learning how and why we see what we do, and consider how the exploration of illusion encourages children's (and adults') curiosity, creativity and confidence (the three ‘Cs'). The chapter is divided into three sections. The first describes the science of seeing illusions: why we see them and what it tells us about how the brain works. The second section – art of seeing – describes one example of applying this scientific understanding in the context of an art project in a primary school classroom. The third section entitled the illusion metaphor explains the importance of using illusions to break with some received methods of learning and teaching by emphasising the ambiguity of learned and inherited truths and conventions, with the implicit aim of encouraging children to respond more empathetically to the world around them. 

Le Martelot, E., Bentley, P. and Lotto, R.B.: Exploiting natural asynchrony and local knowledge within systemic computation to enable generic neural structures. Refereed Proceedings: Proceedings of International Workshop on Natural Computation (IWNC 2007), Nagoya, Japan.
(2007)

Bio-inspired processes are involved more and more in today's technologies, yet their modelling and implementation tend to be taken away from their original concept because of the limitations of the classical computation paradigm. To address this, we previously introduced systemic computation (SC), a model of interacting systems with natural characteristics, and further introduced a modeling platform with a bio-inspired system implementation. In this paper, we investigate the impact of local knowledge and asynchronous computation: signi?cant natural properties of biological neural networks (NN) and naturally handled by SC. We present here a bio-inspired model of arti?cial NN, focussing on agent interactions, and show that exploiting these built-in properties, which come for free, enables neural structure ?exibility without reducing performance.

Le Martelot, E., Bently, P.J. and Lotto, R.B.: A systemic computation platform for modelling and analysis of processes with natural characteristics. Refereed Proceedings: Proceedings of Genetic and Evolutionary Computation Conference (GECCO 2007), London, UK.
(2007)

Computation in biology and in conventional computer architectures seem to share some features, yet many of their important characteristics are very different. To address this, (Bentley 2007) introduced systemic computation, a model of interacting systems with natural characteristics. Following this work, here we introduce the first platform implementing such computation, including programming language, compiler and virtual machine. To investigate their use we then provide an implementation of a genetic algorithm applied to the travelling salesman problem and also explore how SC enables self-adaptation with the minimum of additional code.

Download

Malkin, D. and Lotto, R.B.: Evolutionary benefits of evolvable component integration. Refereed Proceedings: Proceedings of Genetic and Evolutionary Computation Conference (GECCO 2007), London, UK.
(2007)

A system composed of multiple interacting components is capable of responding to contextual information and can produce a higher range of non-linear responses to stimuli compared to a modular system with a low degree of component interaction. However, the fitness landscape of highly integrated systems is more rugged indicating that such systems are likely to be less evolvable. In this work we use an artificial life simulation to investigate whether the evolvability of highly integrated systems can be improved if the level of integration between the system's components is under evolutionary control. When evolving our multi-component system we discover that the level of integration very quickly falls to virtually zero reducing the ruggedness of the landscape and making it nearly neutral. This allows the evolving population to explore the genome space without getting stuck on local optima. The components then integrate and the evolving population settles on the global optimum. This work is unique because the presented problem requires the evolving system to be fully integrated in order to solve it and as such the decreased ruggedness and near neutrality are not a permanent feature of the landscape but rather a property which is temporarily manipulated and exploited by the evolving population. 

Download

Corney, D. and Lotto, R.B.: What are lightness illusions and why do we see them? Public Library of Science Computational Biology 3:e180.
(2007)

Lightness illusions are fundamental to human perception, and yet why we see them is still the focus of much research. Here we address the question by modelling, not human physiology or perception directly as is typically the case, but our natural visual world and the need for robust behaviour. Artificial neural networks were trained to predict the reflectance of surfaces in a synthetic ecology consisting of 3-dimensional ‘dead-leaf' scenes under non-uniform illumination. The networks learned to solve this task accurately and robustly given only ambiguous sense data. In addition – and as a direct consequence of their experience – the networks also made systematic ‘errors' in their behaviour commensurate with human illusions, which includes brightness contrast and assimilation – although assimilation (specifically White's illusion) only emerged when the virtual ecology included 3D, as opposed to 2D scenes. Subtle variations in these illusions, also found in human perception, were observed, such as the asymmetry of brightness contrast. These data suggest that ‘illusions' arise in humans because (i) natural stimuli are ambiguous, and (ii) this ambiguity is resolved empirically by encoding the statistical relationship between images and scenes in past visual experience. Since resolving stimulus ambiguity is a challenge faced by all visual systems, a corollary of these findings is that human illusions must be experienced by all visual animals regardless of their particular neural machinery. The data also provide a more formal definition of illusion: the condition in which the true source of a stimulus differs from what is its most likely (and thus perceived) source. As such, illusions are not fundamentally different from non-illusory percepts, all being direct manifestations of the statistical relationship between images and scenes. 

Download

Hulme, D.J., Hirsch, R., Buxton, B.F. and Lotto, R.B.: A new reduction from 3SAT to n-partite graphs. Refereed Proceedings: IEEE.
(2007)

The Constraint Satisfaction Problem (CSP) is one of the most prominent problems in artificial intelligence, logic, theoretical computer science, engineering and many other areas in science and industry. One instance of a CSP, the satisfiability problem in propositional logic (SAT), has become increasingly popular and has illuminated important insights into our understanding of the fundamentals of computation. Though the concept of representing propositional formulae as n-partite graphs is certainly not novel, in this paper we introduce a new polynomial reduction from 3SAT to Gn7 graphs and demonstrate that this framework has advantages over the standard representation. More specifically, after presenting the reduction we show that many hard 3SAT instances represented in this framework can be solved using a basic arc-consistency algorithm, and finally we discuss the potential advantages and implications of using such a representation. 

Download

Understanding and Realizing Presence in the Presenccia Project.
(2006)

Slater,M., Frisoli,A., Tecchia,F., Guger,C., Lotto,B., Steed,A., Pfurtscheller,C., Leeb,R., Reiner,M., Sanchez-Vives,M.V., Bernardet,U., Verschure,P. (2007). Understanding and Realizing Presence in the Presenccia Project. IEEE Computer Graphics and Applications 27(4), 90-93. ISSN: 0272-1716

Howe, Q.H., Lotto, R.B. and Purves, D.: Empirical approaches to visual perception. Journal of Theoretical Biology 241:866-875.
(2006)

Schlessinger, U, Peter Bentley, and Lotto, R.B.: Investigating the emergence of multicellularity using a population of neural network agents. PPSN IX 4193:711-720.
(2006)

This paper expands Mosaic World, an artificial life model, in order to directly test theories on the emergence of multicellular life. Five experiments are conducted and demonstrate that both the presence of predation and accidental aggregation are sufficient conditions for the transition to multicellularity. In addition, it is shown that division of labour is a major benefit for aggregation, and evolves even if aggregates ‘pay' for abilities they do not use. Analysis of evolved results shows multiple parallels to natural systems, such as differentiation in constituent members of an aggregate, and life-like, complex ecosystems. 

Download

Schlessinger, U, Peter Bentley, and Lotto, R.B.: Evolving visually guided agents in an ambiguous virtual world. Refereed Proceedings: Proceedings of Genetic and Evolutionary Computation Conference (GECCO 2005), Washington, D.C.
(2005)

The fundamental challenge faced by any visual system within natural environments is the ambiguity caused by the fact that light that falls on the system's sensors conflates multiple attributes of the physical world. Understanding the computational principles by which natural systems overcome this challenge and generate useful behaviour remains the key objective in neuroscience and machine vision research. In this paper we introduce Mosaic World, an artificial life model that maintains the essential characteristics of natural visual ecologies, and which is populated by virtual agents that – through ‘natural' selection – come to resolve stimulus ambiguity by adapting the functional structure of their visual networks according to the statistical structure of their ecological experience. Mosaic World therefore presents us with an important tool for exploring the computational principles by which vision can overcome stimulus ambiguity and usefully guide behaviour. 

Download

Schlessinger, U, Peter Bentley, and Lotto, R.B.: Analysing the efficacy of structure mutations on the evolvability of A-Life neural network agents. Refereed Proceedings: Proceedings of the European Conference on Artificial Life (ECAL 2005), Canterbury, UK.
(2005)

This paper investigates evolvability of artificial neural networks within an artificial life environment. Five different structural mutations are investigated, including adaptive evolution, structure duplication, and incremental changes. The total evolvability indicator, Etotal, and the evolvability function through time, are calculated in each instance, in addition to other functional attributes of the system. The results indicate that incremental modifications to networks, and incorporating an adaptive element into the evolution process itself, significantly increases neural network evolvability within open-ended artificial life simulations. 

Download

Lotto, R.B. and Chittka, L.: Seeing the light: Illumination as a contextual cue to color choice behavior in bumblebees. Proceedings of the National Academy of Sciences, USA 102:3852-3856.
(2005)

The principle challenge faced by any color vision system is to contend with the inherent ambiguity of stimulus information, which represents the interaction between multiple attributes of the world (e.g., object reflectance and illumination). How natural systems deal this problem is not known, though traditional hypotheses are predicated on the idea that vision represents object reflectance accurately by discounting early in processing the conflating effects of illumination. Here we test the merits of this general supposition by confronting bumblebees (Bombus terrestris) with a color discrimination task that can only be solved if information about the illuminant is not discounted, but maintained in processing, and thus available to higher-order learned behavior. We show that bees correctly use the intensity and chromaticity of illumination as a contextual cue to guide them to different target colors. In fact, we trained bees to choose opposite – rather than most similar – target colors after an illumination change. This performance cannot be explained with a simple color constancy mechanism that discounts illumination. Further tests show that bees do not use a simple assessment of the overhead illumination, but the spectral relationships between a floral target and its background. These results demonstrate that bees can be ‘color constant' without discounting the illuminant; that in fact they can use the illumination itself as a salient source of information. 

Download

Lotto, R.B. and Wicklein, M.: Bees encode behaviourally significant spectral relationships in complex scenes to resolve stimulus ambiguity. Proceedings of the National Academy of Science USA 102:16870-16874.
(2005)

Bees like humans can continue a surface from its colour even when the scene's global illuminant changes (which is a phenomenon called colour constancy). It is not known, however, whether they can also generate colour constant behaviour in more natural complex scenes that are lit by multiple lights simultaneously, conditions in which most computational models of colour constancy fail. To test this, bumblebees were raised in a highly controlled, yet ecological relevant environment consisting of a matrix of 64 artificial flowers under four spatially distinct lights. As in nature, the bees had no direct access to information about the illuminants or flowers. Furthermore, the background of all the flowers in the matrix was black, independent of illumination. The stimulus information presented to the bee was, therefore, far more constrained than that normally experienced in nature. Despite this, the bees learned to identify the rewarded flowers in each differently illuminated region of the matrix, even when the illumination of one of the regions was switch with one not previously experienced. These behavioural results suggest that colour constant behaviour is not resolved by simply adapting to the global average of spectral stimulus, nor even the spectral contrast between an object and its immediate surround, but can use behaviorally relevant contrast relationships between statistically dependent, but visually distinct stimulus elements of scenes. 

Download

Lotto, R.B. and Purves, D.: Perceiving Colour. Colour Dyes Review.
(2005)

Purves, D., Williams, S.M, Nundy, S. and Lotto, R.B.: Perceiving the intensity of light. Psychological Reviews 111:142-158.
(2004)

Lotto, R.B. and Price, D.J.: Development of the nervous system. In Medical Physiology, Eds. Ellory, C., Nye, P., Morris, J., Miall, C. and Blakemore, C. Mosby Yearbook Europe.
(2004)

Andrews, T. and Lotto, R.B.: Perceptual rivalry is contingent on the perceptual meaning of stimuli Current Biology. 14:418-423.
(2004)

We view the world with two eyes and yet are typically only aware of a single, coherent image. Arguably the  simplest explanation for this is that the visual system unites the two monocular stimuli into a common stream  that eventually leads to a single coherent sensation. However, this notion is inconsistent with the well known phenomenon of rivalry; when physically different stimuli project to the same retinal location, the ensuing perception alternates between the two monocular views in space and time [2]. Although fundamental for understanding the principles of binocular vision and visual awareness, the mechanisms underlying binocular rivalry remain controversial. Specifically, there is uncertainty about what determines Physically Identical Targets whether monocular images undergo fusion or rivalry. By taking advantage of the perceptual phenomenon of color contrast, we show that physically identical monocular stimuli tend to rival—not fuse when they signify different objects at the same location in visual space. Conversely, when physically different monocular stimuli are likely to represent the same object at the same location in space, fusion is more likely to result. The data suggest that what competes for visual awareness in the two eyes is not the physical similarity between images but the similarity in their perceptual/empirical meaning. 

Download

Lotto, R.B.: Visual development: Experience puts the colour in life. Current Biology 14: pp. R619-R621.
(2004)

Recent findings show that colour processing, like most other sensory attributes, is shaped by experience. While such studies can reveal the mechanisms of development, can they also help uncover the mechanisms of perception? 

Download

Haynes, J., Lotto, R. B. and Rees, G.: Responses of human visual cortex to uniform surfaces measured with fMRI. Proceedings of the National Academy of Science USA 101:4286-4291.
(2004)

Surface perception is fundamental to human vision, yet most studies of visual cortex have focused on the processing of borders. We therefore investigated the responses of human visual cortex to parametric changes in the luminance of uniform surfaces by using functional MRI. Early visual areas V1 and V2􏰀V3 showed strong and reliable increases in signal for both increments and decrements in surface luminance. Responses were significantly larger for decrements than for increments, which was fully accounted for by differences in retinal illumination arising from asymmetric pupil dynamics. Responses to both sustained and transient changes of illumination were transient. Signals in early visual cortex scaled linearly with the magnitude of change in retinal illumination, as did subjects' subjective ratings of the perceived brightness of the stimuli. Our findings show that early visual cortex responds strongly to surfaces and that perception of surface brightness is compatible with brain responses at the earliest cortical stages of processing. Thus, there could be import ant interactions between regions representing the surface and those representing the border. To ensure that the responses we measured were caused by the local surface alone rather than remote contours, we studied the represent action of parts of a surface that were separated from the closest contour by at least 5°. This distance is beyond the influence of boundary processing that has been measured in human V1, V2, and V3. 

Download

Asavaritikrai, P., Lotto, R.B., Anderson, G. and Price, D.J.: Regulation of cell survival in the developing thalamus; an in vitro analysis Experimental Neurology 181:39-46.
(2003)

There is evidence that developing thalamic cells become dependent for their survival on the integrity of their afferent and/or efferent connections, which may provide required levels of neural activity and/or essential neurotrophic factors. These connections develop in the second half of gestation in mice and, during this time (embryonic days 17–19), isolated thalamic cells either grown as explants or dissociated from each other lose their ability to survive. Here we show that the loss of viability of explants, but not of dissociated cells, is delayed if the cultures are treated with depolarizing stimuli. The survival of dissociated thalamic cells is promoted by culture medium conditioned by thalamic explants grown with depolarizing stimuli, indicating that the effect of depolarization involves trophic factors released by thalamic cells. This survival promoting effect is found prenatally, but not postnatally, and is prevented by the neurotrophin blocker K252a. Culture medium conditioned by cortex also promotes the survival of thalamic cells and this effect does occur postnatally. These findings suggest that diffusible factors, possibly members of the neurotrophin family, and depolarizing stimuli regulate thalamic cell survival before birth, but trophic support from cortex becomes crucial after birth. This culture model may provide a means of investigating the mechanisms of thalamic cell survival during development. 

Download

Lotto, R.B. and Purves, D.: Why we see what we do. Optician 225:22-26.
(2003)

7. Purves, D. and Lotto, R.B.: The Cornsweet Effect. 'Encyclopaedia of Neuroscience'.
(2003)

Purves, D.P. and Lotto, R.B.Why we see what we do: A wholly probabilistic strategy of vision. Sinaur Associates INC. (Sunderland Massachusetts) and Macmillan Press (London, UK).
(2003)

This book describes an empirical theory for why we see illusions.

Download

Purves, D., Lotto, R.B., and Nundy, S.: From the cover: Why we see what we do. American Scientist 90:236-243.
(2002)

Lotto, R.B. and Purves, D.: From the cover: A rationale for the structure of colour space. Trends in Neuroscience 25:82-86.
(2002)

The colors perceived by humans in response to light stimuli are generally described in terms of four color categories (reds, greens, blues and yellows), the members of which are systematically arrayed around gray. This broadly accepted description of color sensation differs fundamentally from the light that induces it, which is neither ‘circular' nor categorical. What, then, accounts for these discrepancies between the structure of color experience and the physical reality that underlies it? We suggest that these differences are based on two related requirements for successful color vision:(1) that spectra be ordered according to their physical similarities and differences; and (2) that this ordering be constrained by the four-color map problem. 

Download

Lotto, R.B. and Purves, D.: The empirical basis of color perception. Consciousness and Cognition 11:609-629.
(2002)

Purves, D., Lotto, R.B., Williams, S.M., Nundy, S. and Yang, Zhiyong: Why we see things the way we do: Evidence or a wholly empirical strategy of vision. Philosophical Transactions of the Royal Society 356:285 – 297.
(2001)

Lotto, R.B., Asavaritidrai, P., Vali, L. and Price, D.J.: Mechanisms regulating programmed cell death in the developing forebrain. Journal of Neuroscience 21:3904-3910.
(2001)

Many neurons die as the normal brain develops. How this is regulated and whether the mechanism involves neurotrophic molecules from target cells are unknown. We found that cultured neurons from a key forebrain structure, the dorsal thalamus, develop a need for survival factors including brain-derived neurotrophic factor (BDNF ) from their major target, the cerebral cortex, at the age at which they innervate it. Experiments in vivo have shown that rates of dorsal thalamic cell death are reduced by increasing cortical levels of BDNF and are increased in mutant mice lacking functional BDNF receptors or thalamocortical projections; these experiments have also shown that an increase in the rates of dorsal thalamic cell death can be achieved by blocking BDNF in the cortex. We suggest that the onset of a requirement for cortex-derived neurotrophic factors initiates a competitive mechanism regulating programmed cell death among dorsal thalamic neurons. 

Download

Lotto, R.B. and Purves, D.: From the cover: An empirical explanation of the Chubb illusion. Journal of Cognitive Neuroscience 13:547-555.
(2001)

The perceived difference in brightness between elements of a patterned target is diminished when the target is embedded in a similar surround of higher luminance contrast (the Chubb illusion). Here we show that this puzzling effect can be explained by the degree to which imperfect transmittance is likely to have affected the light that reaches the eye. These observations indicate that this 'illusion' is yet another signature of the fundamentally empirical strategy of visual perception, in this case generated by the typical influence of transmittance on inherently ambiguous stimuli. 

Download

Purves, D., Lotto, R.B. and Polger, T.: From the cover: Color vision and the Four-Color-Map problem. Journal of Cognitive Neuroscience 12:233-237.
(2000)

Four different colors are needed to make maps t hat avoid adjacent countries of the same color. Because the retinal i mage is two dimensional, like a map, four dimensions of chromatic experience would also be needed to optimally distinguish therefore suggest that the organization of human color vision according to four-color classes (reds, greens, blues, and yellows) has arisen as a solution to this logical requirement in topology. 

Download

Nundy, S., Lotto, R.B., Coppola, D, Shimpi, A. and Purves, D.: Why are angles misperceived? Proceedings of the National Academy of Science USA 97:5592-5597.
(2000)

Although it has long been apparent that observers tend to overestimate the magnitude of acute angles and underestimate obtuse ones, there is no consensus about why such distortions are seen. Geometrical modeling combined with psychophysical testing of human subjects indicates that these misperceptions are the result of an empirical strategy that resolves the inherent ambiguity of angular stimuli by generating percepts of the past significance of the stimulus rather than the geometry of its retinal projection. 

Download

Lotto, R.B. and Purves, D.: From the cover: An empirical explanation of color contrast. Proceedings of the National Academy of Science USA 97:12834-12839.
(2000)

For reasons not well understood, the color of a surface can appear quite different when placed in different chromatic surrounds. Here we explore the possibility that these color contrast effects are generated according to what the same or similar stimuli have turned out to signify in the past about the physical relationships between reflectance, illumination, and the spectral returns they produce. This hypothesis was evaluated by (i) comparing the physical relationships of reflectances, illuminants, and spectral returns with the perceptual phenomenology of color contrast and (ii) testing whether perceptions of color contrast are predictably changed by altering the probabilities of the possible sources of the stimulus. The results we describe are consistent with a wholly empirical explanation of color contrast effects. 

Download

Purves, D., Williams, M. and Lotto, R.B. The relevance of visual perception to cortical evolution and development. In: Evolution Developmental Biology of the Cerebral Cortex. Novartis foundation Symposia. John Wiley & Sons, New York, pp. 240-258.
(2000)

Lotto, R.B., Williams, S.M. and Purves, D.: An empirical basis for Mach bands. Proceedings of the National Academy of Science USA 96: pp. 5239-5244.
(1999)

If Mach bands arise as an empirical consequence of real-world luminance profiles, several predictions follow. First, the appearance of Mach bands should accord with the appearance of naturally occurring high-lights and lowlights. Second, altering the slope of an ambiguous luminance gradient so that it corresponds more closely to gradients that are typically adorned with luminance maxima and minima in the position of Mach bands should enhance the illusion. Third, altering a luminance gradient so that it corresponds more closely to gradients that normally lack luminance maxima and minima in the position of Mach bands should diminish the salience of the illusion. Four, the perception of Mach bands elicited by the same luminance gradient should be changed by contextual cues that indicate whether the gradient is more or less likely to signify a curved or a f lat surface. Because each of these predictions is met, we conclude that Mach bands arise because the association elicited by the stimulus (the percept) incorporates these features as a result of past experience. 

Download

Lotto, R.B., Williams, S.M. and Purves, D.: Mach bands as empirically derived associations. Proceedings of the National Academy of Science USA 96: pp. 5245-5250.
(1999)

Mach bands, the illusory brightness maxima and minima perceived at the initiation and termination of luminance gradients, respectively, are generally considered a direct perceptual manifestation of lateral inhibitory interactions among retinal or other lower order visual neurons. Here we examine an alternative explanation, namely that Mach bands arise as a consequence of real-world luminance gradients. In this first of two companion papers, we analyze the natural sources of luminance gradients, demonstrating that real-world gradients arising from curved surfaces are ordinarily adorned by photometric highlights and lowlights in the position of the illusory bands. The prevalence of such gradients provides an empirical basis for the generation of this perceptual phenomenon.  

Download

Lotto, R.B., Price, D.J., Upton, L. and Gaspar, P: 5HT receptor activation enhances neurite outgrowth of thalamic neurones. Neuroscience Letters 269:87-90.
(1999)

Download

Purves, D., Shimpi, A. and Lotto, R.B.: From the cover: An empirical explanation of the Cornsweet effect. Journal of Neuroscience. 19:8542-8551.
(1999)

A long-standing puzzle in vision is the assignment of illusory brightness values to visual territories based on the characteristics of their edges (the Craik–O'Brien–Cornsweet effect). Here we show that the perception of the equiluminant territories flanking the Cornsweet edge varies according to whether these regions are more likely to be similarly illuminated surfaces having the same material properties or unequally illuminated surfaces with different properties. Thus, if the likelihood is increased that these territories are surfaces with similar reflectance properties under the same illuminant, the Craik–O'Brien–Cornsweet effect is diminished; conversely, if the likelihood is increased that the adjoining territories are differently reflective surfaces receiving different amounts of illumination, the effect is enhanced. These findings indicate that the Craik–O'Brien–Cornsweet effect is determined by the relative probabilities of the possible sources of the luminance profiles in the stimulus. 

Download

Lotto, R.B. and Purves, D.: From the Cover: The effects of color on brightness. Nature Neuroscience 2:1010-1014.
(1999)

Observation of human subjects shows that the spectral returns of equiluminant colored surrounds govern the apparent brightness of achromatic test targets. The influence of color on brightness provides further evidence that perceptions of luminance are generated according to the empirical frequency of the possible sources of visual stimuli, and suggests a novel way of understanding color contrast and constancy. 

Download

Cooke, S., Grant, G., McLauchlan, C., Lotto, R.B. and Price, D.J.: Basic fibroblast growth factor promotes subplate survival in explant cultures of embryonic mouse cortex. Neuroscience Letters 271:143-146.
(1999)

Lotto, R.B., Aitkenhead, A. and Price, D.J.: Effects of the thalamus on the development of cerebral cortical efferents in vitro. Journal of Neurobiology vol. 39: pp. 186-196.
(1999)

Download

Lotto, R.B. and Price, D.J. Cultures and co-cultures of thalamic nuclei. In The Neuron in Culture. IBRO Methods in the Neurosciences.
(1998)

Lotto, R.B. and Price, D.J. Primary cultures of thalamic neurons. In The Neuron in Culture. IBRO Methods in the Neurosciences.
(1998)

Lotto, R.B., Clausen, J.A. and Price, D.J.: A role for neurotrophins in the survival of murine embryonic thalamic neurons. European Journal of Neuroscience vol. 9: pp. 1940-1949.
(1997)

Price, D.J. and Lotto, R.B.: Influences of the thalamus on the survival of subplate and cortical plate cells in cultured embryonic mouse brain Journal of Neuroscience vol. 16: pp. 3247-3255.
(1996)

Lotto, R.B. and Price, D.J.: Effects of subcortical structures on the growth of cortical neurites in vitro. NeuroReport vol. 7: pp. 1185-1188.
(1996)

Dutia, M.B., Lotto, R.B. and Johnston, A.R.: Time course of maturation of membrane properties of mouse Type A and Type B medial vestibular nucleus neurones. Acta Otolaryngol (Stockh). vol. 520: pp. 101-104.
(1995)

Lotto, R.B. and Price, D.J.: The stimulation of thalamic neurite outgrowth by cortex-derived growth factors in vitro: the influence of cortical age and activity. European Journal of Neuroscience vol. 7: pp. 318-328.
(1995)

Price, D.J., Lotto, R.B., Warren, N., Magowan, G. and Clausen, J. The roles of growth factors and neural activity in the development of the neocortex. Proc. of the CIBA Symp., vol. 193; pp. 231-244.
(1995)

Lotto, R.B.: An in vitro search for factors controlling the formation and maintenance of connections between the thalamus and cortex. PhD Thesis University of Edinburgh.
(1994)

Rennie, S., Lotto, R.B. and Price, D.J.: Growth-promoting interactions between the murine neocortex and thalamus in organotypic co-cultures. Neuroscience vol. 61: pp. 547-564.
(1994)

Lotto, R.B. and Price, D.J.: Evidence that molecules influence axonal growth and termination in the developing geniculocortical pathway are conserved between divergent mammalian species. Developmental Brain Research vol. 81: pp. 17-25.
(1994)